Air Hammer Bits

Percussion air hammers have been used for decades in shallow air drilling operations. These shallow operations have been directed at the drilling of water wells, monitoring wells, geotechnical boreholes, and mining boreholes. In the past decade, however, the percussion air hammers have seen increasing use in drilling deep oil and natural gas wells. Percussion air hammers have a distinct advantage over roller cutter bits in drilling abrasive, hard rock formations.

The use of percussion air hammers (or down-the-hole air hammers) is an acceptable option to using rotating tri-cone or single cone drill bits for air and gas drilling operations. The air hammer utilizes an internal piston (or hammer) that is actuated by the compressed air (or other gas) flow inside the drill string. The internal piston moves up and down in a chamber under the action of air pressure applied either below or above the piston through ports in the inside of the air hammer. In the downward stroke, the hammer strikes the bottom of the upper end of the drill bit shaft (via a coupling shaft) and imparts an impact load to the drill bit. The drill bit in turn transfers this impact load to the rock face of the bit. This impact load creates a crushing action on the rock face very similar to that discussed above at the beginning of Section 3.2.2. But in this situation, the crushing action is dynamic and is more effective than the quasi-static crushing action of tri-cone and single cone drill bits. Therefore, air hammer drilling operations require far less WOB as comparable drilling operations using tri-cone or single cone drill bits.

The air hammer is made up to the bottom of the drill string and at the bottom of the air hammer is the air hammer bit. The air hammer drill string must be rotated just like a drill string that utilizes tri-cone or single cone drill bits. The rotation of the drill string allows the inserts (i.e., tungsten carbide studs) on the bit face to move to a different location on the rock face surface. This rotation allows a different position on the rock face to receive the impact load as the upper end of the hammer bit is struck by the hammer. In direct circulation operations, air flow passes through the hammer section, through the drill bit channel and orifices to the annulus. As the air passes into the annulus, the flow entrains the rock cuttings and carries the cuttings to the surface in the annulus. Direct circulation air hammers are available in a wide variety of outside housing diameters (3 inches to 16 inches). These air hammers drill boreholes with diameters from 3 5/8 inches to 17 1/2 inches.

There are also reverse circulation air hammers. These unique air hammers allow air pressure in the annulus to actuate the hammer via ports in the outside housing of the hammer. The reverse circulation air hammer bits are designed with two large orifices in the bit face that allow the return air flow with entrained rock cuttings to flow to the inside of the drill string and then to the surface. Reverse circulation air hammers are available in larger outside housing diameters (6 inches to 24 inches). These air hammers drill boreholes with diameters from 7 7/8 inches to 33 inches. Figure 3-11 shows two typical air hammer bits that would be used with direct circulation air hammers. The larger bit (standing on its shank end) is an 8 5/8 inch

Large Diameter Hammer Bits
Figure 3-11: Two typical air hammer bits with concave face (8 5/8 inch diameter bit on end, 6 inch diameter on side) (courtesy of Rock Bit International Incorporated).

diameter concave bit. The smaller bit (laying on its side) is a 6 inch diameter concave bit.

There are five air hammer bit cutting face designs. Figure 3-12a shows the profile of the drop center bit and Figure 3-12b shows the profile of the concave bit. Figure 3-13a shows the profile of the step gauge bit and Figure 3-13b shows the profile of the double gauge bit. Figure 3-14 shows the profile of the flat face bit. These five bit cutting face designs are applicable for a variety of drilling applications from non-abrasive, soft rock formations to highly abrasive, very hard rock formations. The application of these five face designs are shown in Figure 3-15.

Air Hammer Drill Bit Sandvik Drills
Figure 3-12: Air hammer bit face profile designs, a) drop center bit, and b) concave bit (courtesy AB Sandvik Rock Tools).
Sandvik Rock Drill
Figure 3-13: Air hammer bit face profile designs, a) step gauge bit, and b) double gauge bit (courtesy AB Sandvik Rock Tools).

Figure 3-14: Air hammer bit face profile design, flat face bit (courtesy AB Sandvik Rock Tools).

In the past the air hammer manufactures have provided the air hammer bits for their specific air hammers. This practice insured compatibility of bit with hammer housing. The increased air hammer use in drilling deep oil and natural gas recovery wells has attracted traditional oil field drill bit manufacturers to fabricate air hammer bits. Although the air hammer bit faces are somewhat uniform in design, the shafts are different for each air hammer manufacturer. The air hammer face and shafts are integral to the bit, thus, manufacturing air hammer bits is complicated. Fortunately, the air hammer has proven in the past decade to be very effective in drilling deep boreholes. This has given rise to competition among traditional drill bit manufacturers to provide improved air hammer bits for deep drilling operations. This competition has in turn resulted in an increase in the quality and durability of air hammer bits (over the traditional air hammer manufacturer-supplied air hammer bits) in the more hostile environments of the deep boreholes. Operational use of the air hammer will be discussed in detail in Chapter 11.

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Rock Formation Abrasiveness

Figure 3-15: Air hammer bit face profile designs and application to rock formation abrasiveness and hardness (courtesy AB Sandvik Rock Tools).

Figure 3-15: Air hammer bit face profile designs and application to rock formation abrasiveness and hardness (courtesy AB Sandvik Rock Tools).

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